[1] Field of the Invention
The present invention relates to alkaline storage batteries including nickel-cadmium storage batteries and nickel-hydrogen storage batteries, and lithium ion storage-batteries and other nonaqueous storage batteries; and a method of manufacturing these storage batteries. The present invention pertains, in particular, to a processing technique for a part of the electrode substrate to which the current collector is joined.
[2] Related Art
In recent year, alkaline storage batteries—such as nickel-cadmium storage batteries and nickel-hydrogen storage batteries, and nonaqueous storage batteries as represented by lithium ion storage batteries having high energy densities are used as electric sources for portable devices (e.g. laptop personal computers) and electric power tools, and furthermore for vehicles (e.g. electric-assisted bicycles, hybrid bikes and hybrid bicycles).
Among the above-mentioned batteries, storage batteries intended for the use of vehicles and electric power tools are required to have high output characteristics and a high energy density. Therefore, regarding the storage batteries for these usages, it is essential to enhance the current collecting efficiency by reducing resistance of the current collecting components and forming a tight joint between the current collector and the electrode substrate of each polar plate. Additionally, as to the storage batteries for vehicles and electric power tools, vibration stemming from the power source or the like of a device in a driving state is transferred also to the storage batteries. It is accordingly necessary for such storage batteries to have high joint strength between the current collector and the electrode substrate of the substrate so that the current collector and the substrate will not come apart from each other as a result of the vibration transmitted from the power source of the device.
A typical structure of alkaline storage batteries, nonaqueous storage batteries and the like is that: a positive and a negative electrodes opposing one another with a separator sandwiched therebetween are spirally wound; disk-shaped current collectors are respectively joined to a part of each electrode substrate (i.e. a tab portion) of the positive and negative electrodes, which projects from a different one of the two side edges of the separator; and all the components in this condition are housed in a case. The case for the storage batteries is composed of a cylindrical case having a bottom and a sealing cap mounted to seal the open end of the case. The positive-electrode current collector connected to the positive electrode is connected to either one of the case or the sealing cap while the negative-electrode current collector is connected to the other.
In late years, foamed nickel has been used as electrode substrates for certain types of storage batteries in order to increase the manufacturing efficiency. However, an electrode substrate made of foamed nickel is as it is difficult to form a joint with a current collector since it is highly porous and has a small density. A usual method adopted to overcome such a difficulty is to weld in advance a ribbon-shaped tab on an edge of the electrode substrate to which the current collector is desired to be joined. Another method with a view to further enhancing the manufacturing efficiency is to compress a portion of the electrode substrate where the current collector is to be joined, and join the current collector to the electrode substrate without using a ribbon-shaped tab, as disclosed in Japanese Laid-Open Patent Application Publication No. S62-136759, for example. The method disclosed in this publication is described next with the aid of FIG. 1A.
Being an electrode substrate made of foamed nickel, a positive electrode 511 is filled with an active material entirely, except for one part. To be more specific, the positive electrode 511 has, in the width direction of the electrode substrate (i.e. the Y-axis direction), (i) an active material filled portion 511a that is filled with the active material and (ii) an active material unfilled portion 511b that is not filled with the active material, as shown in FIG. 1A. The active material unfilled portion 511b corresponds to part of the positive electrode 511 to which the current collector is to be joined (a tab portion). As shown in the magnified image in FIG. 1A, compression is applied to the active material unfilled portion 511b in the width direction of the electrode substrate (i.e. the Y-axis direction) Here, compression is applied so that the length of the active material unfilled portion 511b in the width direction of the positive electrode 511 is reduced to, for example, one-tenth. The above-stated patent publication proposes a technique that increases the density of the active material unfilled portion 511b in the electrode substrate by compression, and thus enables the current collector to be joined to the electrode substrate in a reliable manner while eliminating the need of a ribbon-shaped tab.
When an electrode assembly 510 is formed by winding the above-mentioned electrode 511 together with a negative electrode 512 and a separator 513, however, it is sometimes the case that the separator 513 becomes positionally misaligned in the Z-axis direction (hereinafter, referred to as “winding misalignment”), as shown in FIG. 1B (portion E in FIG. 1B). In the case when the winding misalignment of the separator 513 is created, a misaligned portion 513a of the separator 513, sticking out above the upper edge of the active material unfilled portion 511b of the positive electrode 511 in the Z-axis direction, becomes tucked between the current collector and the positive electrode 511 when the current collector is joined to the positive electrode 511, which possibly results in joint failure. Furthermore, it is required to reduce the size of the active material unfilled portion 511b which does not contribute to the generation of electricity, especially in the present time where an increase in the energy density of storage batteries is desired. Such a reduction in the size further increases the chance of the occurrence of joint failure.
In order to reliably achieve joining of the current collector even when the winding misalignment of the separator 513 has bee created, a method may be adopted in which the relative position of the separator 513 to the positive electrode 511 in the Z-axis direction is changed, as shown in FIG. 2, at the formation of an electrode assembly 515. In such a structure, the upper edge of the separator 513 may be set lower, by a distance t54, than the boundary between the active material filled portion 511a and active material unfilled portion 511b of the positive electrode 511, which thus allows a reliable joint of the current collector even if a misaligned portion 513b of the separator 513 is created as indicated by portion F in FIG. 2. As has been already mentioned above, however, further enhancement of the energy density is desired for alkaline storage batteries, nonaqueous storage batteries and the like. Accordingly, it is not acceptable to increase a portion making no contribution to the electric generation (i.e. a portion where the positive and negative electrodes 511 and 512 do not oppose each other with the separator 513 sandwiched therebetween), as in the case of the electrode assembly 515.
In the case where the relative position of the separator to the positive electrode 511 is changed in order to join the current collector in a reliable fashion even when winding misalignment of the separator 513 has been created, a portion of the positive electrode 511 not covered by the separator 513 increases (by an amount corresponding to the distance t54), as shown in FIG. 2. As a result, the following disadvantages can be observed: vibration is added to the storage battery; the storage battery becomes subject to damage when dropped or the like happens—for example, part of the positive electrode 511 bends with the impact; and the portion of the positive electrode 511 not covered by the separator 513 causes loss of the active material. Given these issues, it is considered that adopting a structure as shown in FIG. 2 causes a decrease in the storage capacity of the battery, and at the same time, makes the storage battery prone to causing an internal short circuit in the event of vibration being applied, or when the storage battery is dropped, for example.